So what are Gravity Waves

Two black holes, when they met, cause a cavity
In the space time contin’um called gravity
And the waves, fast as light,
Prove that Einstein was right,
As the scientists claim, with audacity.

The science involves much complicated mathematics but a simple model will explain the basic idea.

Think of space as a large rubber sheet with the planets and stars resting on it.  Heavier object depress the surface more than lighter ones, which are attracted to the larger holes. What we call gravity is really a measure of how deep a particular hole is. Very heavy stars sink so low that they appear to make a hole so deep that there seems to be no bottom. These holes are what are called "black holes" and they are so deep that even light cannot get out.
When two black holes collide enormous amount of energy is released and this causes the sheet to vibrate violently. These ripples speed out all over the universe at the speed of light.
Such events are such a long way away (billions of light years) that they are extremely small by the time they reach the Earth.
Because the effects on Earth are so small they are would be impossible to measure with a conventional tape measure but in the LIGO project a laser beam is bounced back and forth in a long tube to produce the equivalence of a very very long measuring rod, Sophisticated electronics means that minute changes in the length of this laser "tape" can be measured, Because the equipment is so sensitive other things might give a false reading so there are two units, one in Louisiana and the other in Washington, and the only changes that are of interest are those which occur in both instruments at the same time.
The excitement in the media yesterday was that it has been announced that gravity waves have been detected for the first time and the shape of the waves fits the predictions made by Einstein a hundred years ago.